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LT1934-1 Datasheet(PDF) 7 Page - Linear Technology |
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LT1934-1 Datasheet(HTML) 7 Page - Linear Technology |
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7 / 20 page  LT1934/LT1934-1 7 1934fe OPERATION The LT1934 uses Burst Mode control, combining both low quiescent current operation and high switching frequency, which result in high efficiency across a wide range of load currents and a small total circuit size. A comparator monitors the voltage at the FB pin of the LT1934. If this voltage is higher than the internal 1.25V reference, the comparator disables the oscillator and power switch. In this state, only the comparator, reference and undervoltage lockout circuits are active, and the current into the VIN pin is just 12μA. As the load current discharges the output capacitor, the voltage at the FB pin falls below 1.25V and the comparator enables the oscillator. The LT1934 begins to switch, delivering current to the output capaci- tor. The output voltage rises, and when it overcomes the feedback comparator’s hysteresis, the oscillator is disabled and the LT1934 returns to its micropower state. The oscillator consists of two one-shots and a flip-flop. A rising edge from the off-time one-shot sets the flip-flop, which turns on the internal NPN power switch. The switch remains on until either the on-time one-shot trips or the current limit is reached. A sense resistor and amplifier monitor the current through the switch and resets the (Refer to Block Diagram) flip-flop when this current reaches 400mA (120mA for the LT1934-1). After the 1.8μs delay of the off-time one- shot, the cycle repeats. Generally, the LT1934 will reach current limit on every cycle—the off time is fixed and the on time is regulated so that the LT1934 operates at the correct duty cycle. The 1.8μs off time is lengthened when the FB pin voltage falls below 0.8V; this foldback behavior helps control the output current during start-up and overload. Figure 1 shows several waveforms of an LT1934 producing 3.3V from a 10V input. When the switch is on, the SW pin voltage is at 10V. When the switch is off, the inductor current pulls the SW pin down until it is clamped near ground by the external catch diode. The switch driver operates from either the input or from the BOOST pin. An external capacitor and diode are used to generate a voltage at the BOOST pin that is higher than the input supply. This allows the driver to fully saturate the bipolar switch for efficient operation. If the SHDN pin is grounded, all internal circuits are turned off and VIN current reduces to the device leakage current, typically a few nA. VOUT 50mV/DIV VSW 10V/DIV Figure 1. Operating Waveforms of the LT1934 Converting 10V to 3.3V at 180mA (Front Page Schematic) 1934 F01a ISW 0.5A/DIV ILI 0.5A/DIV 5μs/DIV |
Similar Part No. - LT1934-1_15 |
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Similar Description - LT1934-1_15 |
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